The index $i$ is an integer from $1$ to $\left\lfloor x\right\rfloor $
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Américo TavaresOct 31 '10 at 21:41

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Presumably you replace $\sum_{i=1}^{x} x + \sum_{i=1}^{x} 2$ with $x(x-1) + 2(x-1)$ because you think each part is equal - but it is easy to see that $\sum_{i=1}^{x} x$ is not equal to $x(x-1)$ just by trying a values of $x$. You can always check your steps numerically to try and find where exactly the discrepancy creeps in.
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anonOct 31 '10 at 21:48

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Thank you everyone for the insight, I'm trying to get better at math :)) It's all right now.
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QosmoOct 31 '10 at 21:50